The disclosures of the following priority applications are herein incorporated by reference:
Japanese Patent Application No. 2000-109776 filed on Apr. 11, 2000; and
Japanese Patent Application No. 2001-83559 filed on Mar. 22, 2001.
1. Field of the Invention
The present invention relates to a wide converter lens for shortening the focal length and widening the angle of view of a photographic lens system including thereof by attaching to an object side of the photographic lens system, in particular, relates to a wide converter lens capable of obtaining the angle of view of about 80xc2x0 or over.
2. Description of Related Art
Wide converter lenses used by attaching to an object side of a photographic lens system have been proposed in large number. Above all, as a wide converter lens capable of obtaining the angle of view of about 80xc2x0 or over, a wide converter lens is disclosed in Japanese Laid-Open Patent Application No. 11-305118.
However, since an example disclosed in Japanese Laid-Open Patent Application No. 11-305118 produces distortion more than 10% when the wide converter lens is applied, the appearance of distortion in a photographed scene has sometimes become obvious according to circumstances.
The present invention is made in view of the aforementioned problem and has an object to provide a wide converter having an angle of view of about 80xc2x0 or over with fewer appearance of distortion.
According to one aspect of the present invention, a wide converter lens used by attaching to an object side of a photographic lens includes, in order from the object side, a first positive lens having double convex surfaces, a negative lens unit including at least one negative lens, and a second positive lens having double convex surfaces. The following conditional expression is satisfied:
xe2x88x920.8 less than (r2+r1)/(r2xe2x88x92r1) less than 0.8xe2x80x83xe2x80x83(1) 
where r1 denotes the radius of curvature of the object side surface of the first positive lens and r2 denotes the radius of curvature of the image side surface of the first positive lens.
In one preferred embodiment of the present invention, the following conditional expression is satisfied;
45 less than "ugr"1xe2x80x83xe2x80x83(2) 
where "ugr"1 denotes Abbe number of the first positive lens.
Conditional expression (1) defines an appropriate shape of the first positive lens. When the value (r2+r1)/(r2xe2x88x92r1) falls below the lower limit of conditional expression (1), aberrations such as astigmatism and coma deteriorate severely, so it is disagreeable. On the other hand, when the value exceeds the upper limit of conditional expression (1), correction of distortion becomes difficult.
Conditional expression (2) defines an appropriate range of Abbe number of the first positive lens. When the Abbe number falls below the lower limit of conditional expression (2), correction of lateral chromatic aberration becomes difficult.
In one preferred embodiment of the present invention, the negative lens unit consists only of a first negative lens with a meniscus shape having a convex surface facing to the object side. In this construction, since the wide converter lens can be constructed with small number of lens element, it is advantageous to lower the manufacturing cost.
In one preferred embodiment of the present invention, the negative lens unit consists of, in order from the object side, a first negative lens with a meniscus shape having a convex surface facing to the object side, and a second negative lens with a meniscus shape having a convex surface facing to the object side. This construction is advantageous for correcting coma, so that more preferable correction can be realized.